This invention relates generally to a structural vessel as well as to an apparatus and a method of forming same. More particularly, the vessel is double-walled, is wound helically and has a helically wound stiffening rib.
Long cylindrical vessels of large diameter are known for use as platform supports for offshore oil drilling operations, and are normally fabricated from a plurality of cylindrical plate sections seamed welded along contiguous or overlapping edges between sections to form a single-walled or a double-walled vessel. Circular reinforcing ribs are needed as well as longitudinal ribbing, and sometimes spaced bulkheads are required to insure structural integrity. The fabrication of especially such large and heavy vessels is, however, extremely time consuming and costly requiring numerous labor-consuming steps during the welding and weld inspection operations.
On the other hand, cylinder conduits, tubes and piping have been fabricated more quickly and less laboriously by helically winding sheets of material and sealing together contiguous edges or surfaces of each pair of successive windings. Normally, however, such a helical seam pipe or tube is designed principly for use as a single-walled conduit for fluids (pressurized or not), communication lines, and the like, requiring a relatively small diameter of sufficient shell strength without additional reinforcements.